JP2023164061A - Mgb2 superconducting wire rod, method of manufacturing mgb2 superconducting wire rod, superconducting coil and magnetic generator - Google Patents

Abstract

To provide a MgB2 superconducting wire rod capable of reducing a flexure radius to a practical value without largely reducing a critical current, and capable of further long sizing or thinning, a method of manufacturing the MgB2 superconducting wire rod, a superconducting coil therewith, and a magnetic generator therewith.SOLUTION: A superconducting wire rod 200 includes: a group of wires in which a plurality of wires 201 containing MgB2 are arranged in the circumferential direction and radial direction with respect to a center of the wire rod; a high thermal expansion metal 204 arranged so as to cover the wires and having a coefficient of thermal expansion at a room temperature higher than the wire 201; a stabilizing material 205 arranged so as to cover the high thermal expansion metal 204 to stabilize the superconduction; and a hard metal 206 having a higher hardness than the stabilizing material 205 arranged so as to cover the stabilizing material 205. A method for manufacturing a superconducting wire rod includes: forming a built-in material in which precursors of a plurality of wires 201 are accommodated in a multilayer tube, drawing the built-in material, and heat-treating the built-in material to form MgB2.SELECTED DRAWING: Figure 2

Description

The present invention relates to an MgB ₂ superconducting wire using magnesium diboride (MgB ₂ ), a method for manufacturing the MgB ₂ superconducting wire, a superconducting coil, and a magnetic generator.

The advantage of superconducting wires is that they allow current to flow with zero resistance. Conventionally, wires made of niobium titanium (NbTi), which is a low-temperature superconductor, have been widely used for superconducting coils. A superconducting coil using NbTi wire has a low operating temperature of about 4K, so it is cooled with liquid helium. However, in recent years, there has been concern about tight supply and demand for helium. Therefore, efforts are being made to develop superconductors that have a high critical temperature and do not use liquid helium, and to make them into wires.

Niobium tin (Nb ₃ Sn), yttrium (Y)-based oxides, bismuth (Bi)-based oxides, magnesium diboride (MgB ₂ ), and the like are known as superconductors with high critical temperatures. MgB ₂ has a high critical temperature of about 39K, and raw materials are relatively easy to obtain. It also has small magnetic anisotropy, is lightweight, and has excellent mechanical properties. Therefore, MgB ₂ superconducting wires using MgB ₂ as a superconductor are expected to be applied to various uses.

As a general method for manufacturing MgB ₂ superconducting wire, a powder in tube (PIT) method is used in which a metal tube is filled with powder as a raw material and the metal tube is drawn. PIT methods include ex situ methods and in situ methods. The ex situ method is a method using pre-synthesized MgB ₂ as a raw material. The in situ method is a method in which magnesium powder and boron powder are used as raw materials, and magnesium and boron are heat-treated to generate _MgB2 .

Methods for producing superconducting coils using _MgB2 superconducting wire include a wind-and-react method and a react-and-wind method. The wind-and-react method is a method in which a precursor of a superconducting wire is wound into a coil shape and then heat-treated. The react-and-wind method is a method in which heat-treated superconducting wire is wound into a coil.

It is known that MgB ₂ superconducting wire has a permissible bending radius, and its superconducting properties deteriorate if it is bent with a large curvature. If the MgB ₂ superconducting wire is bent at a large curvature below the allowable bending radius during winding into a coil shape, routing, etc., superconducting properties such as critical current will deteriorate. Due to such characteristics, there are currently restrictions on the production of superconducting magnets using MgB ₂ superconducting wires.

Patent Document 1 describes a filament formed of _MgB2 , a base material covering the outer periphery of the filament, a high thermal expansion metal covering the outer periphery of the base material, and a stabilizing material for stabilizing superconductivity covering the outer periphery of the high thermal expansion metal. A wire rod with the following is described. A high thermal expansion metal is defined as a metal whose coefficient of thermal expansion at room temperature is higher than that of MgB ₂ and the base material.

JP 2021-106079 Publication

In Patent Document 1, compressive residual stress is applied to the MgB ₂ filament using a high thermal expansion metal to increase the resistance of the wire against bending. By improving the bending strain resistance of the wire, the allowable bending radius is reduced while ensuring a critical current. However, as a result of studies conducted by the present inventors, it was confirmed that there is room for further improvement in the bending strain resistance of the MgB ₂ superconducting wire and in making the wire thinner and longer.

Therefore, the present invention provides an MgB ₂ superconducting wire that can reduce the bending radius to a realistic value without greatly reducing the critical current, and that enables further lengthening and thinning of the wire _. The present invention aims to provide a method for manufacturing a superconducting wire, a superconducting coil using the same, and a magnetism generating device equipped with the same.

In order to solve the above problems, a superconducting wire according to the present invention is an MgB ₂ superconducting wire in which a plurality of strands containing MgB ₂ are covered with a metal layer, and the plurality of strands are arranged with respect to the center of the wire. A group of wires arranged in a circumferential direction and a radial direction, a high thermal expansion metal having a coefficient of thermal expansion at room temperature higher than that of the wires, which is arranged so as to cover the group of wires, and a metal that covers the high thermal expansion metal. A stabilizing material that stabilizes superconductivity is arranged, and a high-hardness metal that is arranged to cover the stabilizing material and has a hardness higher than that of the stabilizing material is provided.

Further, the method for manufacturing a superconducting wire according to the present invention is a method for manufacturing a MgB ₂ superconducting wire in which a plurality of strands containing MgB ₂ are covered with a metal layer, wherein a precursor of the plurality of strands is a multilayer tube. forming an embedded material housed in the embedded material; a step of wire-drawing the embedded material; and a step of heat-treating the wire-drawn embedded material to generate _MgB2 . The built-in material includes a plurality of precursors of the strands arranged in the circumferential direction and radial direction with respect to the center of the multiple tube and accommodated in the multiple tube, and the multiple tube has a coefficient of thermal expansion at room temperature. A high thermal expansion metal tube made of a high thermal expansion metal whose hardness is higher than that of the wire, a stabilizer tube made of a stabilizing material that stabilizes superconductivity, and a high hardness metal whose hardness is higher than that of the stabilizing material. The high-hardness metal tubes made of are arranged in this order from the inside to the outside.

According to the present invention, it is possible to reduce the bending _radius to a realistic value without greatly reducing the critical current, and the MgB ₂ superconducting wire can be made even longer and thinner. A method for manufacturing a wire, a superconducting coil using the same, and a magnetism generator equipped with the same can be provided.

FIG. 1 is a cross-sectional view schematically showing an example of an embedded material that is a precursor of a MgB ₂ superconducting wire according to an embodiment of the present invention. FIG. 1 is a cross-sectional view schematically showing an example of a MgB ₂ superconducting wire according to an embodiment of the present invention. 1 is a cross-sectional view schematically showing an example of an embedded material that is a raw material for an MgB ₂ superconducting wire according to an embodiment of the present invention. FIG. 1 is a cross-sectional view schematically showing an example of a MgB ₂ superconducting wire according to an embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing an example of a built-in material that is a precursor of a conventional MgB ₂ superconducting wire. FIG. 1 is a cross-sectional view schematically showing an example of a superconducting coil according to an embodiment of the present invention. 1 is a cross-sectional view schematically showing an example of a magnetism generating device according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a MgB ₂ superconducting wire, a method for manufacturing the MgB ₂ superconducting wire, a superconducting coil using the same, and a magnetism generator equipped with the same will be described with reference to the drawings, according to an embodiment of the present invention. In addition, in each of the following figures, common components are given the same reference numerals and redundant explanations will be omitted.

FIG. 1 is a cross-sectional view schematically showing an example of an embedded material that is a precursor of a MgB ₂ superconducting wire according to an embodiment of the present invention.
As shown in FIG. 1, the MgB ₂ superconducting wire according to the present embodiment is manufactured using as a precursor an assembly material 100 incorporating a wire precursor 101 which is a precursor of a superconducting filament. The incorporating material 100 is formed by incorporating a plurality of wire precursors 101 into a metal multi-tube (104, 105, 106) in a regular arrangement. The wire precursor 101 is formed by filling a metal tube 103 with raw material powder 102 .

FIG. 2 is a cross-sectional view schematically showing an example of the MgB ₂ superconducting wire according to the embodiment of the present invention.
As shown in FIG. 2, the MgB ₂ superconducting wire 200 according to the present embodiment includes a plurality of wires 201 that are superconducting filaments, a matrix (base material) 202 in which the wire groups are embedded, and a metal layer ( 204, 205, 206).

The MgB ₂ superconducting wire 200 shown in FIG. 2 is manufactured by drawing the embedded material 100 shown in FIG. 1 and then subjecting it to heat treatment. The method for manufacturing the MgB ₂ superconducting wire 200 according to the present embodiment includes a step of forming an embedded material 100 that is a precursor of the MgB ₂ superconducting wire, a step of wire-drawing the embedded material 100, and a step of wire-drawing the embedded material 100. The method includes a step of heat-treating the assembled material 100 to generate _MgB2 .

The MgB ₂ superconducting wire 200 according to the present embodiment is a superconducting wire in which a plurality of wires 201 containing MgB ₂ are covered with a metal layer (204, 205, 206) having a multilayer structure. This MgB ₂ superconducting wire 200 has a multifilamentary wire structure including a plurality of strands 201 that are superconducting filaments. The plurality of wires 201 are arranged in the circumferential direction and the radial direction with respect to the center of the wire to form a wire group. The matrix 202 has a group of wires embedded inside the metal layer (204, 205, 206).

The metal layers (204, 205, 206) are provided in a multilayer structure in which a high thermal expansion metal 204, a stabilizing material 205, and a high hardness metal 206 are arranged in this order from the inside to the outside. The high thermal expansion metal 204 is formed of the high thermal expansion metal tube 104. Stabilizer 205 is formed from stabilizer tube 105 . The high hardness metal 206 is formed of the high hardness metal tube 106.

According to the MgB ₂ superconducting wire 200 according to this embodiment, since the high hardness metal 206 is provided on the outside of the stabilizing material 205, problems during wire drawing of the embedded material 100 are prevented. The high hardness metal 206 prevents uneven deformation of the high thermal expansion metal 204 and the stabilizing material 205 that occurs during wire drawing. Therefore, compared to the conventional method, it is possible to make the wire material longer and thinner. Even though a multifilamentary wire structure in which the wires 201 are regularly arranged is adopted, deviation of the wire groups is prevented.

The strand precursor 101 is a single core wire for assembly which is a precursor of the strand 201, and is made by housing a raw material powder 102 containing magnesium and boron in a metal tube 103. The wire 201, which is a superconducting filament, is formed by a powder in tube (PIT) method. The PIT method is a method of filling a metal tube with raw material powder and subjecting the metal tube to wire drawing to produce a wire rod. When the embedded material 100 is wire-drawn and then heat-treated, the raw material powder 102 becomes a wire 201 containing MgB ₂ , and the metal tube 103 becomes a matrix 202 .

As a method for forming the wire 201 containing MgB ₂ , either an ex situ method or an in situ method among PIT methods may be used, but it is preferable to use an in situ method. According to the in situ method, MgB ₂ with many bonds between particles and few voids can be produced by heat treatment at a relatively low temperature. When MgB ₂ is fired at a low temperature, the density of grain boundaries that become pinning centers increases, so superconducting properties such as critical current density can be improved.

A carbon source can be added to the raw material powder 102 containing magnesium and boron. When a carbon source is added, B in MgB ₂ can be replaced by C during the heat treatment to generate MgB ₂ . Since C serving as an impurity is introduced into the superconductor, the critical current and critical magnetic field of the MgB ₂ superconducting wire 200 can be improved.

As the carbon source, inorganic carbon compounds such as B ₄ C and SiC, hydrocarbons such as benzene, naphthalene, coronene, and anthracene, organic acids such as stearic acid, and magnesium salts of organic acids can be used.

When the metal tube 103 containing the raw material powder 102 is heat-treated after wire-drawing the embedded material 100, it becomes a matrix 202, which embeds and mechanically supports a group of wires made up of a plurality of wires 201. The metal tube 103 functions as a barrier material during heat treatment to generate _MgB2 . By filling the metal tube 103, which is a barrier material, with the raw material powder 102, reactions between Mg and B contained in the raw material powder 102 and inhibiting factors such as copper are prevented.

As the material of the metal tube 103, iron, niobium, tantalum, nickel, titanium, alloys thereof, etc. can be used. These metals hardly react with Mg or B during heat treatment to generate _MgB2 . Therefore, it is effective as a barrier material without interfering with the production of _MgB2 . The material for the metal tube 103 is preferably iron or niobium. Since iron and niobium have good workability and are relatively inexpensive, the metal tube 103 suitable for wire drawing can be obtained at low cost.

In FIG. 1, the embedded material 100 is provided in a structure in which a plurality of wire precursors 101 are arranged in the circumferential direction and the radial direction with respect to the center of the embedded material 100. A center member 107 is arranged at the center of the built-in member 100. As the core material 107, a metal made of a normal conductor metal that does not become a superconducting filament is arranged.

When such a built-in material 100 is wire-drawn and then heat-treated, a group of wires is formed in which a plurality of wires 201 are arranged in the circumferential direction and the radial direction with respect to the center of the wire, as shown in FIG. . A matrix 202 made of metal is formed at the center of the MgB ₂ superconducting wire 200.

As shown in FIG. 1, when a core material 107 made of metal is placed at the center of the embedded material 100, the precursor group composed of a plurality of wire precursors 101 can be mechanically arranged by the core material 107. can be supported. Therefore, during the wire drawing process of the embedded material 100, it is possible to suppress the strand precursor 101 from being biased in the radial direction of the embedded material 100.

As the material of the core material 107, iron, niobium, tantalum, nickel, titanium, alloys thereof, etc. can be used. It is preferable that the core material 107 is made of the same metal as the metal tube 103 containing the raw material powder 102. If they are made of the same type of metal, the processing strain difference and thermal expansion difference between the core material 107 and the metal tube 103 will be small. During wire drawing or heat treatment, uneven force due to plastic deformation or thermal deformation is less likely to be applied to the wire precursor 101, so that the wire precursor 101 is prevented from being biased in the radial direction of the embedded material 100. can do.

In FIG. 1, the plurality of wire precursors 101 are arranged around the core material 107 on a first concentric circle concentric with the core material 107. Further, the plurality of wire precursors 101 are arranged on a second concentric circle concentric with the core material 107 and outside the first concentric circle. The wire precursors 101 are regularly arranged along the circumferential direction on the first concentric circle and the second concentric circle.

When such a built-in material 100 is wire-drawn and then heat-treated, as shown in FIG. It is formed around 107. Further, a second wire group in which a plurality of wires 201 are arranged on a concentric circle concentric with the core material 107 is formed outside the first wire group.

According to such a multilayered multifilamentary wire structure, the current density of the transport current per wire can be increased while reducing AC loss and stabilizing superconductivity. In the case of a multifilamentary wire structure with a multilayer structure, there is a possibility that the strands 201 may be unevenly biased in the cross section of the wire rod when viewed in the axial direction due to the wire drawing process. However, such deviation can be prevented by providing the core material 107 and the high hardness metal 206. Therefore, the MgB ₂ superconducting wire 200 can employ a multifilamentary wire structure with a multilayer structure.

FIG. 3 is a cross-sectional view schematically showing an example of a built-in material that is a raw material for the MgB ₂ superconducting wire according to the embodiment of the present invention.
As shown in FIG. 3, the MgB ₂ superconducting wire according to this embodiment can also be provided in a structure in which a wire precursor, which is a precursor of a superconducting filament, is arranged at the center. The assembly material 300 shown in FIG. 3, like the aforementioned assembly material 100, incorporates a plurality of wire precursors 301 into metal multi-tubes 304, 305, 306 in a regular arrangement. formed by. The wire precursor 301 is formed by filling a metal tube 303 with raw material powder 302 .

The embedded material 300 shown in FIG. 3 differs from the above-described embedded material 100 in that the core material 307 is a precursor of a superconducting filament instead of a metal (107) formed of a normal conductor metal. The point is that the wire precursor 301 is provided. The other main configurations of the built-in material 300 are the same as those of the built-in material 100 described above.

FIG. 4 is a cross-sectional view schematically showing an example of a MgB ₂ superconducting wire according to an embodiment of the present invention.
As shown in FIG. 4, the MgB ₂ superconducting wire 400 according to the present embodiment includes a plurality of strands 401 that are superconducting filaments, a matrix (base material) 402 in which the strands are buried, and a metal layer 404 with a multilayer structure. , 405, 406.

The MgB ₂ superconducting wire 400 shown in FIG. 4 is manufactured by drawing the embedded material 300 shown in FIG. 3 and then subjecting it to heat treatment. The method for manufacturing the MgB ₂ superconducting wire 400 according to the present embodiment includes a step of forming an embedded material 300 that is a precursor of the MgB ₂ superconducting wire, a step of wire-drawing the embedded material 300, and a step of wire-drawing the embedded material 300. The method includes a step of heat-treating the assembled material 300 to generate _MgB2 .

The MgB ₂ superconducting wire 400 according to this embodiment is a superconducting wire in which a plurality of wires 401 containing MgB ₂ are covered with a metal layer (404, 405, 406) having a multilayer structure. This MgB ₂ superconducting wire 400 has a multifilamentary wire structure including a plurality of strands 401 that are superconducting filaments. The plurality of wires 401 are arranged in the circumferential direction and the radial direction with respect to the center of the wire to form a wire group. The matrix 402 has a group of wires embedded inside the metal layer (404, 405, 406).

The metal layers (404, 405, 406) are provided in a multilayer structure in which a high thermal expansion metal 404, a stabilizing material 405, and a high hardness metal 406 are arranged in this order from the inside to the outside. The high thermal expansion metal 404 is formed of the high thermal expansion metal tube 304. Stabilizer 405 is formed from stabilizer tube 305 . The high hardness metal 406 is formed of the high hardness metal tube 306.

In FIG. 3, the embedded material 300 is provided with a structure in which a plurality of wire precursors 301 are arranged in the circumferential direction and the radial direction with respect to the center of the embedded material 300. A center member 307 is arranged at the center of the built-in member 300. As the core material 307, a wire precursor that is a precursor of a superconducting filament is arranged.

When such an embedded material 300 is wire-drawn and then heat-treated, a strand 401, which is a superconducting filament, is formed at the center of the MgB ₂ superconducting wire 400, as shown in FIG. Further, a wire group is formed in which a plurality of wires 401 are arranged in the circumferential direction and the radial direction with respect to the center of the wire.

As shown in FIG. 3, when a core material 307, which is a precursor of a superconducting filament, is placed at the center of the embedded material 300, the wire can be Since the area of the superconducting filament becomes larger in the cross section when viewed in the axial direction, the current density of the transport current per wire can be increased.

In the MgB ₂ superconducting wires 200, 400 according to this embodiment, the number of layers of strands 201, 401 and the number of strands 201, 401 per layer are not particularly limited. The number of layers of wires 201, 401 and the number of wires 201, 401 per layer can be an appropriate number of 2 or more.

As shown in FIGS. 1 and 3, the multiple tubes (104, 105, 106, 304, 305, 306) are made of high thermal expansion metal tubes (104, 304) made of high thermal expansion metal (204, 404), stable The stabilizing material tube (105, 305) formed of the stabilizer material (205, 405) and the high hardness metal tube (106, 306) formed of the high hardness metal (206, 406) are inserted from the inside in this order. It is formed by arranging it outward.

The high thermal expansion metal (204, 404) is provided over the length of the wire so as to cover the outer periphery of the wire group and matrix (202, 402) made up of the wires (201, 401). The high thermal expansion metal is made of a metal whose coefficient of thermal expansion at room temperature is higher than that of the wire or matrix, that is, the material of MgB ₂ or the metal tube that is the barrier material. It is preferable that a high coefficient of thermal expansion is ensured even in the temperature range from the heat treatment temperature for producing MgB ₂ to room temperature and in the extremely low temperature range below the critical temperature of MgB ₂ .

It is generally believed that when a tensile load is applied to a superconducting wire and a critical tensile strain is exceeded, cracks or the like occur in the superconducting filament and the superconducting properties deteriorate. When a superconducting wire is bent during winding into a coil shape, routing, etc., tensile strain occurs on the outside of the bend, and compressive strain occurs on the inside of the bend. When the tensile strain exceeds the residual compressive strain generated during heat treatment, cracks etc. occur in the superconducting filament.

On the other hand, when a high thermal expansion metal is provided, residual compressive strain can be imparted to the superconducting filament by applying a compressive force due to the difference in thermal expansion to the strands after the heat treatment to generate MgB ₂ . As the residual compressive strain increases, the limit tensile strain of the wire is expanded, so the allowable bending radius of the wire can be made smaller than before.

The coefficient of thermal expansion of the high thermal expansion metal at room temperature is preferably 14.0×10 ^-6 °C ^-1 or more, more preferably 14.5×10 ^-6 °C ^-1 or more, and even more preferably 15.0×10 ^-6 ℃ ^-1 or higher. With such a high coefficient of thermal expansion, a sufficiently high residual compressive strain can be imparted to the wire or matrix.

Examples of high thermal expansion metals include stainless steel, carbon steel, nickel steel, nickel chromium steel, and the like. Stainless steel and carbon steel are preferable as the high thermal expansion metal. Stainless steel and carbon steel are inexpensive and have excellent availability, so the material cost for wire rods can be suppressed. Any suitable type of stainless steel or carbon steel can be used as long as the thermal expansion coefficient at room temperature is higher than that of MgB ₂ or the material of the metal tube which is the barrier material, and as long as appropriate workability is ensured.

As the high thermal expansion metal, low carbon stainless steel with a C content of 0.03% by mass or less and low carbon steel with a C content of 0.01% by mass or more and less than 0.25% by mass are more preferable. Specific examples of low carbon stainless steel include SUS301L, SUS304L, and SUS316L. The high thermal expansion metal is preferably a material containing 10% by mass or less of nickel, which inhibits the production of MgB ₂ .

Low carbon stainless steel and low carbon steel are materials with relatively high hardness among stainless steels and carbon steels that have a high coefficient of thermal expansion and a certain degree of ductility. Therefore, if a stabilizing material is sandwiched between low carbon stainless steel or low carbon steel and a high hardness metal, the processing force applied from the outside during wire drawing will cause the high thermal expansion metal and the stabilizing material to adhere to each other with high uniformity. can be done. When the high thermal expansion metal and the stabilizing material are brought into close contact with each other, the difference in deformation between the high thermal expansion metal and the stabilizing material is suppressed during wire drawing. Therefore, compared to cases using low carbon stainless steel or other than low carbon steel, it is possible to make the wire rod longer and thinner. Further, deviation of the wire group in the radial direction is prevented.

The stabilizing material (205, 405) is provided so as to cover the outer periphery of the high thermal expansion metal (204, 404) over the length of the wire. The stabilizing material is made of a good conductor with low resistivity and high thermal conductivity. By providing a stabilizing material, the superconductor can be thermally and magnetically stabilized, and quenching and thermal runaway of the wire can be suppressed.

Copper is preferred as the stabilizing material. Examples of copper include phosphorus deoxidized copper, tough pitch copper, and oxygen-free copper. As copper, oxygen-free copper is particularly preferred. Since oxygen-free copper has high electrical conductivity and high thermal conductivity, it is possible to further improve the thermal stability and magnetic stability of superconductivity.

The high hardness metal (206, 406) is provided so as to cover the outer periphery of the stabilizing material (205, 405) over the length of the wire. The hard metal is formed of a metal whose hardness is higher than that of the stabilizing material. The hardness is preferably ensured at least in the temperature range from the temperature of the heat treatment for producing MgB ₂ to room temperature. The hardness of high-hardness metals can be evaluated, for example, by Vickers hardness (HV).

FIG. 5 is a cross-sectional view schematically showing an example of a built-in material that is a precursor of a conventional MgB ₂ superconducting wire.
As shown in FIG. 5, the conventional MgB ₂ superconducting wire is manufactured using an embedded material 500 having a multifilamentary wire structure as a precursor. The assembly material 500 is formed by incorporating a plurality of wire precursors 501, which are precursors of superconducting filaments, into metal multiple tubes 504 and 505. The wire precursor 501 is formed by filling a metal tube 503 with a raw material powder 502.

In the conventional built-in material 500, multiple tubes 504 and 505 connect a high thermal expansion metal tube 504 made of a high thermal expansion metal and a stabilizing material tube 505 made of a stabilizing material in this order from the inside to the outside. It is formed by arranging it towards the A hard metal tube is not placed on the outside of the stabilizing material tube 505, and the stabilizing material forms the outermost layer.

If the outermost layer of the incorporated material is a stabilizing material as in the past, the outermost stabilizing material receives a direct working force from the outside during wire drawing. Stabilizing materials such as copper are easily deformable and have excellent workability. On the other hand, high thermal expansion metals such as stainless steel are difficult to deform and have poor workability. Therefore, if the outermost layer of the incorporated material is a stabilizing material, the high thermal expansion metal will not be significantly deformed, but the stabilizing material will be significantly deformed.

As a result, a difference in the amount of deformation tends to occur between the high thermal expansion metal on the inside and the stabilizing material on the outside. Only the stabilizing material is greatly reduced in area and is greatly elongated in the wire drawing direction. On the other hand, the high thermal expansion metal and the group of strands inside the metal are not sufficiently reduced in area and do not stretch in the wire drawing direction. When wire-drawing the embedded material, the area reduction rate and wire length reach a plateau, which poses a problem in that converting the embedded material into a wire rod is hindered.

In particular, if a processing force is applied unevenly from the outside to the stabilizing material, only a portion of the stabilizing material in the circumferential direction may be locally thinned. In the cross section of the wire rod when viewed in the axial direction, unevenness may occur at the interface between the high thermal expansion metal and the stabilizing material. If such a difference in the amount of deformation occurs, there is a problem in that the wire precursor becomes biased in the radial direction during wire drawing.

When the biased wire precursor is fired, the wire also becomes biased in the radial direction. When such strands are formed, when the wire is bent, the curvature of some of the strands becomes large. An excessive tensile load is likely to be applied to some of the strands, resulting in cracks and the like, resulting in deterioration of superconducting properties.

On the other hand, if a high hardness metal (206, 406) is provided outside the stabilizing material (205, 405), the difference in deformation that occurs between the high thermal expansion metal and the stabilizing material during wire drawing can be suppressed. be able to. Since there is a high hardness metal on the outside of the stabilizing material, the processing force applied from the outside is applied highly uniformly in the circumferential direction to the wire precursor, high thermal expansion metal, and stabilizing material through the high hardness metal. becomes possible.

Therefore, by providing the high hardness metal (206, 406) on the outside of the stabilizing material (205, 405), the high thermal expansion metal and the stabilizing material can be reduced in area in a well-balanced manner and can be stretched in the wire drawing direction. Compared to the conventional method, it is possible to make the wire longer and thinner within the range where the arrangement of the strands and the superconducting properties are ensured. Moreover, it is possible to prevent deviations in the radial direction from occurring in the arrangement of the wire precursors during wire drawing. Even if a multilayer multifilamentary wire structure is adopted, the allowable bending radius of the wire can be made smaller than before.

The high-hardness metal is preferably a metal whose coefficient of thermal expansion at room temperature is higher than that of the wire or matrix, that is, the material of MgB ₂ or the metal tube that is the barrier material. Further, a metal whose coefficient of thermal expansion at room temperature is higher than that of the stabilizing material is preferable. If the metal has a high coefficient of thermal expansion, a compressive force due to the difference in thermal expansion can be applied to the superconducting filament after heat treatment to generate MgB ₂ .

As the high hardness metal, a copper alloy or an alloy containing copper is preferable. When using a copper alloy or an alloy containing copper, high hardness metal can be easily removed when superconducting wires are connected. When superconducting wires are connected, it is necessary to expose the wires, which are superconducting filaments. However, if the high hardness metal is a copper alloy or an alloy containing copper, it can be easily removed by dissolving it with nitric acid or the like.

As the hard metal, Ni--Cu alloy, Cu--Ni alloy, or dispersion-strengthened copper is more preferable, and Ni--Cu alloy or Cu--Ni alloy is particularly preferable. With these copper alloys, appropriate high hardness can be obtained within the processable range. In addition, an alloy containing Ni is preferable as the outermost layer of the wire because it provides nonmagnetism and corrosion resistance.

Examples of the Cu--Ni alloy include, but are not particularly limited to, cupronickel having a Ni content of 10% by mass or more and 30% by mass or less. Examples of the Ni--Cu alloy include, but are not particularly limited to, Monel and the like having a Cu content of 20% by mass or more and 35% by mass or less. Examples of dispersion-strengthened copper include alumina dispersion-strengthened copper in which alumina is dispersed, zirconia dispersion-strengthened copper in which zirconia is dispersed, and yttria dispersion-strengthened copper in which yttria is dispersed.

After wire drawing, the high thermal expansion metal, the stabilizing material, and the high hardness metal preferably adhere to each other to form an integrated metal layer. Such a metal layer is formed by forming the high thermal expansion metal tube (104, 304), the stabilizing material tube (105, 305) and the high hardness metal tube (106, 306) during the formation of the embedded material (100, 300). They can be formed by arranging them with small gaps between them.

When such an integrated metal layer is formed, the high thermal expansion metal and the stabilizing material can be brought into close contact with each other with high uniformity in the circumferential direction by the processing force applied from the outside during wire drawing. Since only the stabilizing material is prevented from being significantly deformed, the difference in deformation between the high thermal expansion metal and the stabilizing material can be sufficiently suppressed. When forming such an integrated metal layer, it is important that no layer exhibiting other functions is provided between the high thermal expansion metal and the stabilizing material or between the stabilizing material and the high hardness metal. preferable.

Note that in FIGS. 1, 2, 3, and 4, the MgB ₂ superconducting wires 200, 400 and the embedded materials 100, 300 are provided as round wires. However, the MgB ₂ superconducting wires 200, 400 and the embedded materials 100, 300 can be provided in an appropriate cross-sectional shape such as a polygon such as a rectangle or a hexagon, or a flat wire. The number of layers of wires 201, 401 and the number of wires 201, 401 per layer can be an appropriate number of 2 or more.

Next, a method for manufacturing the above-mentioned superconducting wire will be explained. In the following description, a method for manufacturing the MgB ₂ superconducting wire 200 shown in FIG. 2 by an in situ method using the embedded material 100 shown in FIG. 1 will be exemplified.

The MgB ₂ superconducting wire according to the present embodiment includes a preparation process of forming an embedded material, a wire drawing process of wire-drawing the embedded material, and a heat treatment of the wire-drawn embedded material to remove MgB ₂ . and a heat treatment step to generate. In the preparation step, a plurality of single-core wires for assembly are produced as wire precursors for forming a multi-filamentary wire structure. The single core wire for assembly is assembled into a multi-layered tube consisting of a high thermal expansion metal tube, a stabilizer tube and a high hardness metal tube.

In the preparation step, a raw material powder is prepared by mixing magnesium powder and boron powder, and the raw material powder is filled into a metal tube made of a barrier material. The raw material powder is prepared by weighing, pulverizing, and mixing magnesium powder and boron powder, which are raw materials for MgB ₂ , such that the molar ratio of Mg to B is approximately 1:2. A carbon source for element substitution can be added to the raw material powder, if necessary.

The raw material powder is preferably handled in an inert gas atmosphere such as nitrogen or argon, or a non-oxidizing atmosphere such as a vacuum atmosphere. The amount of oxygen and moisture in the atmosphere is preferably 10 ppm or less. The raw material powders can be mixed using a ball mill, a planetary mixer, a V-type mixer, a mortar, or the like.

Further, the raw material powders can be mixed by mechanical milling. In the mechanical milling method, particles of raw material powder are violently collided with media such as zirconia balls or the inner wall of a pot, and are pulverized and mixed while being subjected to strong processing. In mechanical milling, it is preferable to apply collision energy to an extent that does not clearly generate _MgB2 . Note that the production of MgB ₂ can be confirmed by the substantial presence or absence of a MgB ₂ peak in powder X-ray diffraction.

According to the mechanical milling method, B particles penetrate into Mg particles, and a powder structure with a high mixing degree in which B particles are finely dispersed and included in the Mg matrix is obtained. Therefore, when such a powder structure is heat-treated, a superconducting filament with many _MgB2 bonds and few voids can be formed. By forming superconducting filaments with few voids, high critical current densities can be obtained.

Subsequently, the metal tube filled with the raw material powder is wire-drawn to produce a single core wire for assembly. As the single-core wire for assembly, a plurality of wires constituting a group of strands having a multi-core wire structure are produced. It is preferable that the plurality of single-filament wires for assembly be made with the same wire diameter. The wire drawing process of the single core wire for assembly can be performed with an appropriate number of passes. It is preferable to draw the single-filament wire for assembly at an area reduction rate of 8 to 12% per pass.

The wire drawing process of the single core wire for assembly can be performed by drawing process, extrusion process, swaging process, cassette roll process, groove roll process, etc. As the wire drawing device, a draw bench, a hydrostatic extruder, a wire drawing machine, a swager, a cassette roller die, a groove roll, etc. can be used.

Subsequently, a plurality of drawn single-core wires for assembly are assembled together with a core material into a multi-layered tube to produce an assembly material which is a precursor of an MgB ₂ superconducting wire. A multilayer tube consists of a high thermal expansion metal tube made of a high thermal expansion metal, a stabilizing material tube made of a stabilizing material, and a high hardness metal tube made of a high hardness metal in order from the inside to the outside. It is formed by arranging the

When assembling the single-core wires into a multilayer pipe, it is preferable to arrange a plurality of the single-core wires around the center member so as to be concentric with the center member. It is preferable that the individual single core wires for assembly are regularly arranged at equal intervals so as to be line symmetrical with respect to the center line passing through the core material. Further, it is preferable that the core material and the single core wire for assembly, the single core wires for assembly, and the single core wire for assembly and the high thermal expansion metal tube be in contact with each other.

With this arrangement, during wire drawing, processing force can be applied from the high-hardness metal tube to the inner stabilizing material tube and the like with high uniformity in the circumferential direction. Further, after the heat treatment to generate MgB ₂ , compressive force due to the difference in thermal expansion can be uniformly applied to the group of strands from the high thermal expansion metal. During wire drawing, deviation in the radial direction of the arrangement of the wire precursors is suppressed, so excessive load and magnetic loss during bending of the wire can be reduced.

In the wire drawing process, the assembly material in which a plurality of single core wires for assembly are assembled into a multi-layer pipe is wire drawn. By wire-drawing the embedded material at a predetermined area reduction rate, the embedded material is made longer and thinner. The wire drawing process of the incorporated material can be performed with an appropriate number of passes. The wire drawing process of the embedded material 100 can be performed, for example, so that the wire diameter is 0.3 to 2.0 mm. Moreover, the wire diameter can be adjusted according to the purpose.

The wire drawing process of the incorporated material can be performed by drawing process, extrusion process, swaging process, cassette roll process, groove roll process, etc. As the wire drawing device, a draw bench, a hydrostatic extruder, a wire drawing machine, a swager, a cassette roller die, a groove roll, etc. can be used.

Moreover, the wire-drawn embedded material can also be twisted into a spiral shape. If the single core wire for assembly is twisted spirally, the coupling current between the superconducting filaments can be reduced. The twist pitch can be, for example, 10 to 100 mm.

In the heat treatment step, the wire-drawn embedded material is heat treated to generate _MgB2 . When the embedding material is heat-treated at a predetermined temperature or higher, Mg and B in the raw material powder filled into the embedding single-filament wire react, and a wire containing MgB ₂ (MgB ₂ filament) is formed. When the obtained _MgB2 superconducting wire is cooled to room temperature or below the critical temperature after heat treatment, a compressive force is applied to the strands from the high thermal expansion metal due to the difference in thermal expansion, and residual compressive strain is imparted to the _MgB2 filament. Ru.

The heat treatment atmosphere is preferably an inert gas atmosphere such as nitrogen or argon, or a non-oxidizing atmosphere such as a vacuum atmosphere. The amount of oxygen and moisture in the atmosphere is preferably 10 ppm or less. The heat treatment may be performed after the wire-drawn embedded material is wound into a coil shape or the like, or may be performed before being wound into a coil shape or the like. For example, when using a heat-resistant insulating material such as glass fiber, an insulating coating can be applied before heat treatment.

The heat treatment temperature is, for example, 550 to 800°C, preferably 560 to 700°C, more preferably 580 to 620°C. The higher the heat treatment temperature is (550° C. or higher), the more easily the MgB ₂ production reaction proceeds due to the diffusion of Mg. Further, since the thermal expansion of the high thermal expansion metal increases, a large residual compressive strain can be imparted. On the other hand, as the heat treatment temperature is lower (below 800° C.), the grain growth of MgB ₂ is suppressed, so the density of grain boundaries that become pinning centers increases, and a higher critical current density is obtained.

The heat treatment time is, for example, several tens of minutes to several tens of hours, preferably 2 to 16 hours, and more preferably 3 to 12 hours. When the heat treatment time is 3 hours or more, MgB ₂ can usually be sufficiently generated. Moreover, when the heat treatment time is 12 hours or less, the grain growth of MgB ₂ is suppressed, the density of grain boundaries that become pinning centers increases, and a high critical current density is obtained.

According to the MgB ₂ superconducting wire and the manufacturing method of the MgB ₂ superconducting wire according to the present embodiment described above, since the high hardness metal is provided on the outside of the stabilizing material, during the wire drawing process, the high hardness metal that is difficult to deform is used. , machining force can be applied to the inside. Therefore, the high thermal expansion metal and the stabilizing material can be brought into close contact with each other in the circumferential direction with high uniformity. During wire drawing, this prevents only the stabilizing material from greatly reducing its area or from expanding significantly in the drawing direction, suppressing the difference in deformation between the high thermal expansion metal and the stabilizing material. be done. Therefore, compared to the conventional method, it becomes possible to perform wire drawing with a large reduction in area while ensuring the superconducting properties of the wire, and it becomes possible to further lengthen and thin the wire.

Further, since non-uniform deformation of the stabilizing material and the high thermal expansion metal is suppressed, deviation of the strand group in the radial direction is prevented. Even when using a multilayer multifilamentary wire structure, the wire groups are arranged symmetrically with respect to the center of the wire, so when the wire is bent, only some of the wire groups Excessive tensile strain is prevented from occurring. Therefore, the allowable bending radius of the wire can be made smaller than in the past. Since the bending strain resistance of the wire group as a whole is improved, the superconducting properties of the wire can be kept stable even when bending with a small radius is applied.

FIG. 6 is a cross-sectional view schematically showing an example of a superconducting coil according to an embodiment of the present invention.
As shown in FIG. 6, the above-mentioned MgB ₂ superconducting wire comprising a high hardness metal can be used as a superconducting coil 600 wound into a coil shape. A superconducting coil 600 according to this embodiment includes a bobbin 601, the MgB ₂ superconducting wire 602 wound into a coil, and a cooling container 603.

As a method for manufacturing superconducting coil 600, either a wind-and-react method or a react-and-wind method may be used. When using the react-and-wind method, strain is likely to occur in the wire during winding after heat treatment. However, since the above-mentioned MgB ₂ superconducting wire comprising a high-hardness metal has a small allowable bending radius, it is possible to obtain appropriate superconducting properties even if it experiences distortion due to bending compared to the conventional wire.

The bobbin 601 can be made of, for example, a metal with high thermal conductivity. The bobbin 601 is preferably made of copper, and particularly preferably made of oxygen-free copper. If the bobbin 601 has a high thermal conductivity, the MgB ₂ superconducting wire 602 can be cooled with high uniformity. The bobbin 601 is covered with an insulating material (not shown). When using the wind-and-react method, it is preferable to use a heat-resistant material that can withstand heat treatment as the insulating material. Examples of the heat-resistant insulating material include glass braid using glass fiber.

The MgB ₂ superconducting wire 602 can be wound around the bobbin 601. When using the wind-and-react method, the MgB ₂ superconducting wire 602 may be electrically insulated with a heat-resistant insulating material before the heat treatment to generate MgB ₂ . Further, after heat treatment to generate MgB ₂ , it may be impregnated with an insulating resin to be electrically insulated. On the other hand, when using the react-and-wind method, after heat treatment to generate MgB ₂ and winding into a coil shape, it may be impregnated with an insulating resin to be electrically insulated.

The cooling container 603 is a container with a closed structure, and accommodates the MgB ₂ superconducting wire 602 wound into a coil around the bobbin 601 . The cooling container 603 is provided with a structure insulated inside and outside using vacuum insulation, a heat insulating material, a heat shield, or the like. The cooling container 603 may be filled with a cooling medium or may be conductively cooled by a refrigerator.

According to such a superconducting coil, since the MgB ₂ superconducting wire is provided with a high-hardness metal on the outside of the stabilizing material, the permissible bending radius of the wire is smaller than that of the conventional coil. Since bending with a small radius is allowed, it becomes possible to adopt a superconducting coil with a small coil diameter or a structure with a small bending radius at the outlet of the superconducting coil. Therefore, a superconducting coil suitable for downsizing and space saving can be obtained.

FIG. 7 is a cross-sectional view schematically showing an example of a magnetism generating device according to an embodiment of the present invention. FIG. 7 shows an MRI (Magnetic Resonance Imaging) device as an example of a magnetism generating device.
As shown in FIG. 7, the above-mentioned MgB ₂ superconducting wire including a high hardness metal can be provided in an MRI apparatus (magnetism generator) 700 as a superconducting coil 600. The MRI apparatus 700 according to this embodiment includes a static magnetic field generating section 701 configured with the superconducting coil 600 of the MgB ₂ superconducting wire wound into a coil shape.

The MRI apparatus 700 includes a pair of static magnetic field generating sections 701, an imaging region 702, and a gradient magnetic field generating section 703. In FIG. 7, the static magnetic field generating units 701 are connected to each other via a connecting member (not shown). There is. A gradient magnetic field generating section 703 is arranged between the static magnetic field generating section 701 and the imaging region 702, respectively.

The MRI apparatus 700 also includes a bed 705 on which a subject 704 is placed, and a transport mechanism 706 that transports the bed 705. The bed 705 is provided so as to be movable forward and backward relative to the imaging area 702. When the bed 705 is transported by the transport mechanism 706, the subject 704 placed on the bed 705 can move forward and backward with respect to the imaging area 702.

The static magnetic field generating section 701 is composed of the superconducting coil 600 described above. The static magnetic field generating section 701 includes a coil section and a persistent current switch. The coil portion and persistent current switch can be formed from the above-mentioned MgB ₂ superconducting wire. The circuit of the static magnetic field generator 701 is electrically connected to a power source (not shown) via a normal conductor.

An exciting current is passed through the coil section of the static magnetic field generating section 701 when the persistent current switch is OFF. Furthermore, when the persistent current switch is turned on, persistent current is caused to flow. The persistent current flowing through the coil portion generates a static magnetic field with high temporal stability in the imaging region 702. The higher the strength of the static magnetic field, the higher the nuclear magnetic resonance frequency, and therefore the frequency resolution can be improved.

The gradient magnetic field generating unit 703 is supplied with a time-varying current and generates a gradient magnetic field having a spatial distribution in the imaging region 702. When an oscillating magnetic field at a nuclear magnetic resonance frequency is applied to the imaging region 702, a resonance signal is emitted from the subject 704 and is received by a receiving coil (not shown). The received resonance signal is imaged as a magnetic resonance tomographic image of the subject 704 by Fourier transformation. By converting the image into a two-dimensional contrast image or the like, the subject 704 can be inspected and diagnosed.

According to such an MRI apparatus, since it is equipped with a superconducting coil wound with a _MgB2 superconducting wire provided with a high-hardness metal on the outside of a stabilizing material, the permissible bending radius of the wire is smaller than in the past. , it becomes possible to downsize and save space in the magnetic field generating section. Furthermore, since it is possible to adopt a structure in which the bending radius of the exit portion of the superconducting coil is small, multiplexing of the superconducting coils becomes easy.

Although the embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various changes can be made without departing from the spirit of the present invention. For example, the present invention is not necessarily limited to having all the configurations of the embodiments described above. Replacing part of the configuration of one embodiment with another configuration, adding part of the configuration of one embodiment to another form, or omitting part of the configuration of one embodiment Can be done.

For example, the above-mentioned superconducting coil is installed in an MRI apparatus, but the above-mentioned MgB ₂ superconducting wire equipped with a high-hardness metal can be used as a superconducting coil in other devices such as an NMR (Nuclear Magnetic Resonance) apparatus. It may be provided in a magnetism generating device. Other magnetism generators can also be made smaller and space-saving. Furthermore, since the bending radius of the exit portion of the superconducting coil can be made small, multiplexing of the superconducting coils becomes easy.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the technical scope of the present invention is not limited thereto.

<Example 1>
An MgB ₂ superconducting wire having the multifilamentary wire structure shown in FIG. 1 was produced.

First, a single core wire for assembly, which is a precursor of a superconducting filament, was prepared using the following procedure. Raw material magnesium powder and boron powder were weighed so that the molar ratio of Mg to B was about 1:2, and ground and mixed using a ball mill to prepare raw material powder. Then, the obtained raw material powder was filled into an Fe tube.

Subsequently, the Fe tube filled with the mixed powder was wire drawn to obtain a single core wire for assembly. The wire drawing of the Fe tube was performed by drawing using a drawing die. Multiple passes were repeated with an area reduction rate of 8 to 12% per pass. Further, a core material made of the same material as the Fe tube filled with raw material powder was produced by swaging using a swager. Note that the wire diameters of the single core wire for assembly and the core material were the same.

Subsequently, six single-core wires for assembly were arranged so as to surround the periphery of the produced core material, and further, 12 single-core wires for assembly were arranged so as to cover the outer periphery thereof. Then, these single core wire groups for assembly are inserted into a high thermal expansion metal tube, the high thermal expansion metal tube is inserted into a stabilizing material tube, the stabilizing material tube is inserted into a high hardness metal tube, and MgB ₂ An embedded material, which is a precursor of superconducting wire, was obtained.

Note that a SUS316L tube was used as the high thermal expansion metal tube, an oxygen-free copper tube was used as the stabilizing material tube, and a Monel tube was used as the high hardness metal tube.

Subsequently, the fabricated embedded material was wire-drawn to make it thin. The wire drawing process was performed using a drawing die. In the wire drawing process, the wire was thinned by repeating multiple passes with an area reduction rate of 5 to 12% per pass.

Subsequently, the radial cross section of the thinned embedded material was observed using a microscope. As a result, it was confirmed that the inner group of six single-core wires for assembly and the outer group of 12 single-core wires for assembly were arranged regularly in the radial direction, just as they were arranged at the time of insertion. It was done. Furthermore, a multilayer structure was confirmed in which a high thermal expansion metal, a stabilizing material, and a high hardness metal were arranged in this order from the inside to the outside. The high thermal expansion metal and the stabilizing material were reduced to a highly uniform thickness in the circumferential direction. It was confirmed that by placing a high-hardness metal on the outside of the stabilizing material, deformation and irregular arrangement during wire drawing can be reduced.

Subsequently, the wire-thinned embedded material was heat-treated at a temperature of 600° C. to generate MgB ₂ from the raw material powder, thereby manufacturing a MgB ₂ superconducting wire.

<Example 2>
An MgB ₂ superconducting wire having a multifilamentary wire structure shown in FIG. 3 was produced.

First, a single core wire for assembly, which is a precursor of a superconducting filament, was produced in the same manner as in Example 1. The wire drawing process of the metal tube was repeated multiple times with an area reduction rate of 9 to 12% per pass.

Next, six single-core wires were placed to surround the single fabricated single-core wire, and 12 single-core wires were further placed to cover the outer periphery of the fabricated single-core wire. did. Then, as in Example 1, these single core wire groups for assembly are inserted into a high thermal expansion metal tube, the high thermal expansion metal tube is inserted into a stabilizing material tube, and the stabilizing material tube is inserted into a high hardness metal tube. A built-in material, which is a precursor of _MgB2 superconducting wire, was obtained.

Subsequently, the fabricated embedded material was wire-drawn to make it thin. The wire drawing process was performed using a drawing die. In the wire drawing process, the wire was thinned by repeating multiple passes with an area reduction rate of 5 to 12% per pass.

Subsequently, the radial cross section of the thinned embedded material was observed using a microscope. As a result, one single core wire for assembly in the center, a group of six single core wires for assembly on the inside, and a group of 12 single core wires for assembly on the outside are arranged in the radial direction according to the arrangement at the time of insertion. It was confirmed that they were arranged regularly. Furthermore, a multilayer structure was confirmed in which a high thermal expansion metal, a stabilizing material, and a high hardness metal were arranged in this order from the inside to the outside. The high thermal expansion metal and the stabilizing material were reduced to a highly uniform thickness in the circumferential direction. It was confirmed that by placing a high-hardness metal on the outside of the stabilizing material, it was possible to reduce the deformation and disorder of the arrangement of the strands during wire drawing.

Subsequently, the wire-thinned embedded material was heat-treated at a temperature of 600° C. to generate MgB ₂ from the raw material powder, thereby manufacturing a MgB ₂ superconducting wire.

<Comparative example 1>
An MgB ₂ superconducting wire having a multifilamentary wire structure shown in FIG. 5 was produced.

First, a single core wire for assembly, which is a precursor of a superconducting filament, was produced in the same manner as in Example 1. Further, a core material made of the same material as the metal tube filled with raw material powder was produced in the same manner as in Example 1. The wire drawing process of the metal tube was repeated multiple times with an area reduction rate of 9 to 12% per pass.

Next, using the same method as in Example 1, six single core wires for assembly were placed so as to surround the produced core material, and further, 12 single core wires for assembly were placed so as to cover the outer periphery. The core wire was placed. Then, these single core wire groups for assembly were inserted into a high thermal expansion metal tube, and the high thermal expansion metal tube was inserted into a stabilizing material tube to obtain an assembly material which was a precursor of an MgB ₂ superconducting wire.

Note that a SUS316L tube was used as the high thermal expansion metal tube, and an oxygen-free copper tube was used as the stabilizing material tube.

Subsequently, the fabricated embedded material was wire-drawn to make it thin. The wire drawing process was performed using a drawing die. The wire drawing process was repeated multiple times with an area reduction rate of 8 to 12% per pass.

Subsequently, the radial cross section of the thinned embedded material was observed using a microscope. As a result, local thinning was observed only in a part of the circumferential direction of the stabilizing material. Furthermore, the thickness of the stabilizing material was non-uniform, and irregularities were observed at the interface with the high thermal expansion metal. It was confirmed that if a high-hardness metal is not placed outside the stabilizing material, drawing the wire leads to deformation and disordered arrangement of the strands.

As a result of comparison between Examples 1 and 2 and Comparative Example 1, it was found that there is a limit to the lengthening and thinning of the wire when no high-hardness metal is placed outside the stabilizing material. It was confirmed that placing a high-hardness metal on the outside of the stabilizing material makes it possible to make the wire longer and thinner than before.

<Example 3>
A superconducting coil was fabricated using an MgB ₂ superconducting wire having the multifilamentary wire structure shown in FIG.

First, an embedded material was produced in the same manner as in Example 1, and the embedded material was subjected to wire drawing. The assembled material, which had been thinned by wire drawing, was then covered with an insulating material made of glass fiber. Further, the metal bobbin was covered with an insulating material made of glass fiber. Then, the thin wired incorporation material was wound around a bobbin and heat treated at a temperature of 600° C. to generate MgB ₂ . Thereafter, the MgB ₂ superconducting wire was impregnated with an insulating resin and fixed to produce a superconducting coil.

Subsequently, the obtained superconducting coil was placed in a freezing container, and the superconducting coil was electrically connected to a power source. Then, the superconducting coil was excited and the stability of the magnetic field was confirmed.

<Example 4>
An MRI apparatus including a superconducting coil using an MgB ₂ superconducting wire having a multifilamentary wire structure shown in FIG. 1 was manufactured.

The MRI apparatus includes a pair of static magnetic field generators and a gradient magnetic field generator. The static magnetic field generating parts were connected to each other via a connecting member (not shown), and the static magnetic field generating parts were arranged one above the other so as to face each other. A gradient magnetic field generating section was arranged between the static magnetic field generating sections so as to sandwich the imaging region. In addition, a bed and a transport mechanism for transporting the bed were provided so as to be able to move forward and backward with respect to the imaging area.

Subsequently, the obtained superconducting coil was placed in a freezing container and placed in a static magnetic field generator. The superconducting coil was then electrically connected to a power source. It was confirmed that the superconducting coil using the produced MgB ₂ superconducting wire operates normally in an MRI apparatus.

100,300,500... Incorporation material, 101,301,501... Wire precursor, 102,302,502... Raw material powder, 103,303,503... Metal tube, 104,304,504... High thermal expansion metal tube, 105,305,505... Stabilizing material tube, 106,306... High hardness metal tube, 107,307,507... Core material, 200,400... MgB ₂ superconducting wire, 201,401... Element wire, 202,402... Matrix (base material), 204,404...high thermal expansion metal, 205,405...stabilizing material, 206,406...high hardness metal,
600...Superconducting coil, 601...Bobbin, 602... _MgB2 superconducting wire, 603...Cooling container,
700... MRI apparatus (magnetism generator), 701... Static magnetic field generator, 702... Imaging area, 703... Gradient magnetic field generator, 704... Subject, 705... Bed, 706... Transport mechanism

Claims (14)

An MgB ₂ superconducting wire in which a plurality of wires containing MgB ₂ are covered with a metal layer,
a group of wires in which a plurality of the wires are arranged in the circumferential direction and the radial direction with respect to the center of the wire;
a high thermal expansion metal having a higher coefficient of thermal expansion at room temperature than the wires, the metal being disposed so as to cover the group of wires;
a stabilizing material that stabilizes superconductivity and is arranged to cover the high thermal expansion metal;
A MgB ₂ superconducting wire comprising: a high-hardness metal having a hardness higher than that of the stabilizing material, which is disposed so as to cover the stabilizing material. The superconducting wire according to claim 1,
comprising a base material in which the group of wires is embedded;
The high thermal expansion metal is a MgB ₂ superconducting wire, which is a metal whose coefficient of thermal expansion at room temperature is higher than that of the wire and the base material. The superconducting wire according to claim 1,
The high thermal expansion metal, the stabilizing material, and the high hardness metal are in close contact with each other to form _the integrated metal layer. The superconducting wire according to claim 1,
The high thermal expansion metal is a _MgB2 superconducting wire that is low carbon stainless steel or low carbon steel. The superconducting wire according to claim 1,
The stabilizing material is a MgB ₂ superconducting wire made of copper. The superconducting wire according to claim 1,
The high hardness metal is a MgB ₂ superconducting wire, which is a Ni-Cu alloy or a Cu-Ni alloy. The superconducting wire according to claim 1,
comprising a base material in which the group of wires is embedded;
The base material is an MgB ₂ superconducting wire made of iron or niobium. The superconducting wire according to claim 1,
A central material placed at the center of the wire,
A base material in which the wire group and the core material are embedded,
The core material is an MgB ₂ superconducting wire made of the same metal as the base material. The superconducting wire according to claim 1,
Equipped with a center member placed in the center of the wire,
The core material is an MgB ₂ superconducting wire that is a wire containing MgB ₂ . A method for manufacturing an MgB ₂ superconducting wire in which a plurality of wires containing MgB ₂ are covered with a metal layer, the method comprising:
forming a built-in material in which a plurality of the precursors of the strands are housed in a multi-tube;
a step of wire drawing the embedded material;
A step of heat-treating the wire-drawn embedded material to generate _MgB2 ,
The incorporating material is arranged in a circumferential direction and a radial direction with respect to the center of the multiple tube and accommodates the precursors of the plurality of wires in the multiple tube,
The multilayer tube includes a high thermal expansion metal tube made of a high thermal expansion metal whose coefficient of thermal expansion at room temperature is higher than that of the wire, a stabilizing material tube made of a stabilizing material that stabilizes superconductivity, and a hardness A method for manufacturing an MgB ₂ superconducting wire, wherein high-hardness metal tubes made of a metal with higher hardness than the stabilizing material are arranged in this order from the inside to the outside. A method for manufacturing a superconducting wire according to claim 10,
The built-in material has a central material disposed at the center of the multiple pipe,
The precursor of the wire contains powder containing magnesium and boron in a metal tube,
The method for manufacturing an MgB ₂ superconducting wire, wherein the core material is made of the same metal as the metal tube. A method for manufacturing a superconducting wire according to claim 10,
The high thermal expansion metal is low carbon stainless steel or low carbon steel,
the stabilizing material is copper;
The method for manufacturing an MgB ₂ superconducting wire, wherein the high hardness metal is a Ni-Cu alloy or a Cu-Ni alloy. A superconducting coil wound with the superconducting wire according to any one of claims 1 to 9. A magnetism generator comprising a superconducting coil wound with the superconducting wire according to any one of claims 1 to 9.